1. Field of the Invention
The present invention relates to non-fused flux compositions for submerged-arc welding which is performed in various welding positions, more particularly welding flux compositions which are suitable for horizontal submerged-arc welding in production of welded structures such as storage tanks and pressure vessels, and assure good low-temperature fracture toughness of weld material.
Submerged-arc welding is very often used for its high efficiency to achieve horizontal welded joints in welded structures, such as cylindrical tanks, spherical tanks, iron and steel making equipments, such as blast furnaces, hot stoves, reaction vessels in the chemical industry and containers for nuclear reactors.
Meanwhile, in recent years, increased care has been taken with respect to the safety aspect of these steel structures on the basis of the conceptions of fracture mechanics concerning generation of embrittlement fracture, and it is now almost a requisite to assure fracture toughness for these structures. For example, welded joints in LPG (liquefied petroleum gas) tanks have conventionally been specified only by the absorption energy determined by Charpy impact tests, but now they are specified also in respect of the critical COD (Crack Opening Displacement) value, and it is necessary to satisfy the critical COD value.
The horizontal submerged-arc welding, as compared with the welding, in flat position, is limited in the groove opening angle so that slag formed during welding is hard to be detached, and weld beads, due to their gravity, tend to be thin in the upper portions and thick in their lower portions and pock marks and undercuts often occur on the upper side while overlaps and drop through and internal defects such as slag entrappment, pits and blow holes often occur on the lower side.
Further, when a multiple-layer welding is performed using a direct current electric source with a small current, the oxygen content in the resultant weld metal markedly increases and when the welding is performed with an alternating current, the nitrogen content in the weld metal increases, so that it is not possible to obtain good toughness of the weld metal in both cases.
2. Description of Prior Art
Most of conventional flux compositions for submerged-arc welding, particularly fused flux compositions, contain SiO.sub.2 and MnO as main components in order to assure feasibility of their production, and their satisfactory easiness in melting, but these components are unstable oxides which are easily dissociated into oxygen and metals when they are brought into contact with the high-temperature welding arc, so that the slag detachability becomes difficult due to the intersurface energy between the weld metal and the slag, the weld metal tends to entrap the slag and the oxygen content in the weld metal increases, thus lowering the toughness of the weld metal. These problems of the conventional flux compositions are even greater when they are used in horizontal welding operations.
In order to overcome the above problems, flux compositions disclosed in Japanese Patent Publication No. Sho 40-18410, Japanese Patent Publication No. Sho 52-19815 or Japanese Laid-Open Patent Application No. Sho 50-62832 may be used, but it has been found that these flux compositions do not satisfy all of the required Charpy impact value of welded joints, the required critical COD value and the required welding performance in the welded structures as mentioned before where these requirements have been increasingly severe.
For example, the flux composition disclosed in the Japanese Patent Publication No. Sho 40-18410 is designed so as to satisfy both of the required Charpy impact value and the required welding performance, but since the content of SiO.sub.2 is maintained low, and considerable amounts of TiO.sub.2 and CaO are admixed, the slag detachability is deteriorated by the slag sticking to the bead surface due to the precipitation of a high-melting-point complex oxide (CaOTiO.sub.2). Further, although the Charpy impact value at 0.degree. C. is good as the oxygen content in the weld metal lowers, the Charpy impact value sharply decreases at sub-zero temperatures because the micro-structure of the weld metal is not refined so that no practical utility is assured.
Also the flux composition disclosed in the Japanese Patent Publication No. Sho 52-19815 does not provvide a satisfactory stability of the arc, so that bead shapes are markedly disordered, and this flux composition can provide only low values for the Charpy impact value and the critcial COD value at low temperatures.
Further, the flux composition disclosed in the Japanese Laid-Open Patent Application No. Sho 50-62832 can assure to a certain degree the required Charpy impact value at low temperatures, but it contains Ti and B in such a simple manner that the amount of TiO.sub.2 is small, resulting in an insufficient concentration of Ti, and the resultant weld metal is not sufficiently lowered in its oxygen content so that the addition of Ti and B does not fully contribute to the refinement of the weld metal micro-structure and a good critical COD value at low temperatures can not be obtained.